Root rot is primarily caused by prolonged exposure to excess moisture in the soil, creating an anaerobic environment that suffocates the roots. From the plant's perspective, its root system, which is adapted for arid conditions, begins to break down and decay. The pathogen, often a water mold (Phytophthora spp.) or fungus (Fusarium spp.), invades the compromised tissue. The plant cannot uptake water or nutrients effectively, leading to a soft, discolored base (often yellow or brown) and a general collapse of the plant's structure, even though the soil is wet.
This disease is an aggressive infection where bacteria (e.g., Erwinia spp.) or fungi enter through wounds or natural openings. The pathogen secretes enzymes that break down the plant's cell walls, literally digesting the internal tissue. The cactus will appear from the outside as a discolored, water-soaked spot that quickly expands. Internally, the tissue becomes a mushy, often foul-smelling slurry. The plant's vascular system is destroyed, halting all internal transport of water and nutrients and leading to rapid demise.
Sunburn is a physiological disorder, not a pathogenic disease. It occurs when a cactus, especially one not acclimated to intense light, receives more solar radiation than its photosynthetic and protective systems can handle. This damages the chlorophyll and other cellular structures in the epidermis, effectively "cooking" the tissue. The plant responds by forming a layer of scar tissue, which appears as a permanent, bleached, tan, or corky patch. While not directly fatal, severe sunburn creates large dead areas that impair photosynthesis and can serve as entry points for pathogenic infections.
Pests like scale insects and mealybugs are a direct attack on the plant's resources. They use piercing-sucking mouthparts to penetrate the tough skin and feed on the nutrient-rich sap within the vascular system. This drains the cactus of its vital fluids and sugars, weakening it and causing stunted growth, yellowing, and dieback. Furthermore, the honeydew they excrete encourages the growth of sooty mold, which can further inhibit photosynthesis by coating the plant's surface.
Effective treatment involves creating an environment where the plant's natural defenses can operate and where pathogens cannot thrive. For root and soft rots, this means immediate surgery. Removing all infected tissue is crucial to stop the spread, as the plant can compartmentalize and callus over clean wounds. The plant must then be allowed to dry completely in a well-ventilated area to encourage callus formation before being repotted in fresh, sterile, well-draining soil. This new medium provides a physical environment that supports healthy root function and denies pathogens the moisture they need. For sunburn, moving the plant to a location with appropriate light levels prevents further damage, allowing the plant to focus energy on new, healthy growth. Controlling pests through physical removal or horticultural oils suffocates the insects without using harsh chemicals that could further stress the plant, thereby ending the resource drain and allowing for recovery.
